This invention relates to heat treatable, aluminum alloy products. More particularly, it relates to a multiple layer alloy product that is brazeable in a process using a fluoride based flux, one example of which is more commonly known as a Nocolok(copyright) type flux (commonly referred to as the xe2x80x9cNocolok brazing processxe2x80x9d). Nocolok(copyright) is a registered trademark of Alcan Aluminium Ltd of Canada.
Numerous brazeable aluminum alloys have been patented protected. Representative compositions include those taught by U.S. Pat. Nos. 4,040,822, 5,375,760, 5,520,321, 5,535,939, and 5,564,619. Still other aluminum alloys, not specific to brazing, with Nocolok or otherwise, are taught in U.S. Pat. Nos. 2,096,010, 4,589,932, 5,286,445, 5,522,950 and 5,587,029.
To date, it has been difficult to obtain high post-brazing strengths, with tensile yield strengths (or xe2x80x9cTYSxe2x80x9d values) greater than about 90 MPa, for sheet product whose inner and outer surfaces braze well per brazing processes employing Nocolok(copyright) type fluxes. One approach has been to employ an interliner composed of an Aluminum Association (AA) 3003 alloy or 7072 alloy between a 4000 (or 4xxx) series braze cladding and a magnesium-containing, heat treatable core alloy. This interliner primarily acts as a barrier to magnesium diffusion from the core to the braze cladding. In this way, the composite alloy of this invention can obtain higher strengths while still being brazeable by a Nocolok(copyright)-type process.
Known four layer alloy products in this field have predominantly relied on the heat treatable core alloys for elevated, post-brazed strengths. The post braze TYS (tensile yield strength) values of such alloys do not typically exceed about 85 MPa, however, even after extended natural aging times. Such strength depends on the composition of the alloy, aging time and temperature, as well as the cooling rates employed after brazing by a Nocolok(copyright) type brazing process. The ability of manufacturers to subsequently reduce the gauge of these product forms depends strongly on these post braze strengths with higher strengths being clearly more desirable and allowing for greater subsequent gauge reductions.
The present invention relates to a four layer aluminum brazing sheet alloy which may be used as a material in the fabrication of brazed heat exchangers, primarily for folded and/or welded type tubes used in header/tube type heat exchangers (e.g. radiators, heater cores and the like.) The sheet may be fabricated via traditional roll bonding practices or by the practices described in a U.S. patent application filed Oct. 23, 2001 entitled xe2x80x9cSimultaneous Multi-Alloy Castingxe2x80x9d, (inventors Raymond J. Kilmer and J. Lester Kirby), incorporated herein by reference. If this latter practice is used, a thin (less than about 3% of the total composite thickness) divider alloy will be present between at least one and up to three of the interfaces between the four layers described herein. This divider alloy is at least 96% aluminum and serves as a separator/divider to minimize intermixing of the alloys on either side thereof during the casting process. If one or more components of the sheet of the present invention are fabricated via the method of the above-mentioned pending Kilmer et al. patent application, then the sheet will have more than four compositionally distinct layers. The presence of the thin divider alloy does not significantly alter the behavior of the final product nor is it present to intentionally alter the mechanisms described herein. Therefore, all references to four layer products do not necessarily limit the invention to alloys without these dividers. The four layers refer to the compositionally and functionally significant layers of the composite product.
The four layer composite of this invention includes a first interliner positioned between an Aluminum Association (xe2x80x9cAAxe2x80x9d) 4xxx series braze clad alloy and a core alloy and a second interliner positioned on the opposite of the core alloy from the first interliner. At least one of the first and second interliners has higher amounts of Si than that of the core alloy adjacent thereto. The higher Si content interliner preferably contains about 0.02-1.1 wt. % Si, preferably about 0.1-1.1 wt. % Si, most preferably about 0.3-1.1 wt. % Si. The interliners also have low Mg contents to improve the Nocolok brazeability of the product.